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mm: further fix swapin race condition 

Commit 4969c1192d ("mm: fix swapin race condition") is now agreed to
be incomplete.  There's a race, not very much less likely than the
original race envisaged, in which it is further necessary to check that
the swapcache page's swap has not changed.

Here's the reasoning: cast in terms of reuse_swap_page(), but probably
could be reformulated to rely on try_to_free_swap() instead, or on
swapoff+swapon.

A, faults into do_swap_page(): does page1 = lookup_swap_cache(swap1) and
comes through the lock_page(page1).

B, a racing thread of the same process, faults on the same address: does
page1 = lookup_swap_cache(swap1) and now waits in lock_page(page1), but
for whatever reason is unlucky not to get the lock any time soon.

A carries on through do_swap_page(), a write fault, but cannot reuse the
swap page1 (another reference to swap1).  Unlocks the page1 (but B
doesn't get it yet), does COW in do_wp_page(), page2 now in that pte.

C, perhaps the parent of A+B, comes in and write faults the same swap
page1 into its mm, reuse_swap_page() succeeds this time, swap1 is freed.

kswapd comes in after some time (B still unlucky) and swaps out some
pages from A+B and C: it allocates the original swap1 to page2 in A+B,
and some other swap2 to the original page1 now in C.  But does not
immediately free page1 (actually it couldn't: B holds a reference),
leaving it in swap cache for now.

B at last gets the lock on page1, hooray! Is PageSwapCache(page1)? Yes.
Is pte_same(*page_table, orig_pte)? Yes, because page2 has now been
given the swap1 which page1 used to have.  So B proceeds to insert page1
into A+B's page_table, though its content now belongs to C, quite
different from what A wrote there.

B ought to have checked that page1's swap was still swap1.

Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2010-09-19 19:40:22 -07:00 committed by Linus Torvalds
parent 2422084a94
commit 31c4a3d3a0
1 changed files with 5 additions and 3 deletions
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8
mm/memory.c
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@ -2680,10 +2680,12 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
delayacct_clear_flag(DELAYACCT_PF_SWAPIN); delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
/* /*
* Make sure try_to_free_swap didn't release the swapcache * Make sure try_to_free_swap or reuse_swap_page or swapoff did not
* from under us. The page pin isn't enough to prevent that. * release the swapcache from under us. The page pin, and pte_same
* test below, are not enough to exclude that. Even if it is still
* swapcache, we need to check that the page's swap has not changed.
*/ */
if (unlikely(!PageSwapCache(page))) if (unlikely(!PageSwapCache(page) || page_private(page) != entry.val))
goto out_page; goto out_page;
if (ksm_might_need_to_copy(page, vma, address)) { if (ksm_might_need_to_copy(page, vma, address)) {